Molecularly imprinted polymers (MIPs) are polymeric materials containing microscale cavities or imprints of defined shape. To create the imprints, target molecules are introduced into a solution containing polymerizable molecules that bind to the target molecules. Next, reaction conditions are changed, or crosslinking reagents are added to the solution, to cause the polymerizable molecules to form a solid polymer matrix in which the target molecules are immobilized. Finally, the target molecules are removed from the polymer matrix to form imprints having a particular shape. The MIP thus formed is able to selectively bind molecules that match the imprint shape with a lock-and-key-type interaction, when exposed to an environment containing a mixture of compounds.
The most common devices for detecting and quantifying molecules bound to an MIP utilize optical techniques, such as infrared, ultraviolet or visible spectroscopy; chemiluminescence, fluorescence or phosphorescence; or various forms of atomic microscopy. Most of these techniques require large-scale, expensive and technically-complex instrumentation. Analysis of MIPs, using such instruments, is therefore a time-consuming and costly process that is limited to those locations where the instruments are housed. When speed and portability are important because a molecule of interest may decompose or present a hazard, as occurs with many physiologically active compounds, these optical techniques are too slow and awkward to provide meaningful results.
Other detection and quantification devices utilize changes in resistance or capacitance to detect and/or quantify target molecules bound to an MIP. For example, U.S. Pat. No. 6,807,842 discloses a molecular recognition semiconductive polymer sensor system. The system contains carbon or copper doped polymers which are present as a non-imprinted reference polymer and one or more MIP(s). Sampling of an environment with the disclosed system leads to swelling of both the reference polymer and MIP due to absorption of interferents (molecules in the test environment other than target molecules) and target molecules. In particular, the reference polymer absorbs interferents and target molecules into the polymer matrix, whereas the MIP absorbs interferents, but target molecules occupy void spaces provided by the imprints. Thus, target molecules do not contribute to swelling of the MIP. The electrical resistance, which is directly related to distance between dopant atoms, changes in response to the swelling, and the presence and concentration of target molecules in the test environment is determined by the difference in resistance between the reference polymer and the MIP.
There are, however, a number of shortcomings associated with the use of doped polymer composites of the type described above. For example, some dopants, particularly metals, may not be chemically inert, and dopants may slow the release of bound target molecules so that a sensor may not be used for multiple tests in rapid succession or for continuous real-time monitoring of target molecule concentration. Additionally, it is difficult to ensure an even distribution of dopant atoms throughout the polymer matrix during formation of the MIP, and uncontrollable variations in composition make it difficult to reliably reproduce the performance characteristics of the sensors. Further, the repetitious application of electrical current directly to the polymer causes rapid decomposition.